A common method of examining microscopic (including nanometer scale) structures for process monitoring and failure analysis is to cut a trench in the structure with a focused ion beam (FIB) to expose a cross section, and then view the cross section with a scanning electron microscope (SEM). Ion beam milling artifacts, however, can distort the exposed cross section so that the electron beam image does not show an accurate image of the structure.
One artifact is referred to as “curtaining,” because it can look like a curtain. Curtaining occurs when different materials are removed at different rates, such as when the sample is composed of materials that are milled at different rates by the ion beam. Curtaining can also occur when milling a surface that has an irregular shape.
Severe artifacts can be created when exposing a feature having a height that is much greater than its width. Such a structure is referred to as a “high aspect ratio” feature. For example, a feature having a height four times greater than its width would be considered a high aspect ratio feature. For example, holes or contacts between layers in an integrated circuit often have a height that is several times greater than its width.
As semiconductor fabrication processes pack more circuitry into smaller packages, integrated circuit designs are becoming more three-dimensional (3D) and incorporate more high aspect ratio features. In analyzing high aspect ratio structures, especially unfilled contact holes, for 3D integrated circuit (IC) structures such as 3D NAND circuits, conventional ion beam sample preparation causes unacceptable artifacts, such as structure distortion and curtaining.
When there are unfilled high aspect ratio holes on a sample, there are large differences in the milling rates between the solid regions and the regions adjacent to the unfilled hole. The large difference in milling rates results in curtaining or waterfall effects, another artifact that distorts the shape of the hole. Structural damage and artifacts from the ion beam milling process make it difficult to analyze high aspect ratio vertical structures.
For example, one structural feature that process engineers need to observe is a through-silicon via (TSV). Cross-sectioning TSVs is a common practice in semiconductor labs to characterize voids and surface interfaces. Due to the depth of TSVs, typically 50-300 nm, milling a cross section of a TSV with an ion beam can result in substantial curtaining.
Because of the damage and artifacts caused by the use of ion beam milling to expose features, the images do not faithfully show the results of the fabrication process. The artifacts interfere with measurements and with an assessment of the fabrication process because the image and measurements show the results of the sample preparation and not just a product of the manufacturing process.
A method for viewing and measuring high aspect ratio structures without altering the structures or creating artifacts is needed.